What is a Building Envelope?

Building envelope diagram

The building envelope is the primary boundary, between indoors and outdoors. It includes the air barrier, thermal barrier and often the weather barrier. As a system, it offers resistance to soil gases, outdoor temperatures, sunlight, noise, and water. Costly upfront, with lives easily more distant than the occupants.


Illustration of building envelope influences Building envelope diagram Cutaway view of building envelope
Recognize the main boundary Air, thermal, and weather barrier integration Transitions, penetrations, and important connections

Building science research defines this boundary as the most important system in a home. 

  • Indoor Air Quality IAQ
  • Energy Efficiency
  • BuildingScience.com illustration of typical building envelope pressuresComfort
  • Durability

In existing homes, the best ways to improve home performance, is by weatherizing the building envelope. Increasing airtightness and improving insulation details are among the most cost-effective ways of saving energy and is the starting point for better Indoor Air Quality IAQ measurements. For new homes, defining the building envelope at the design stage and ensuring good assembly in the field, is the main opportunity for healthier, higher performing homes.

Building envelope illustration showing physics of natural pressures

There are many hidden forces at work on the building envelope. Vapor and soil gas building pressure from the ground, is one of the most important to control. Physics of wind movement, stack effect and exhaust ventilation exert undesireable influence, increasing risks and reducing effectiveness of poorly installed systems.

Most construction today, continues to ignore the importance of the building envelope. Building codes are beginning to recognize it, and while there is movement in the right direction, most buildings and homes currently being built, fall well short of performance minimums called for by international law.

Antique framing and insulation for your building envelope?

Wood framed construction leaky building envelope illustration.

This speaks to why existing homes have poor indoor air quality, energy efficiency and durability. Unwanted air movement increases energy costs, condensation risks and encourages rain to penetrate the weather barriers.

Leaky building envelopes have less control over indoor-air. They leak too little or too much, often from unhealthy places:

  • Air flowing through hidden building cavities can facilitate mold, mildew and rot.
  • Air from crawlspace or slab: radon, moisture, dust, critter poop, mold.
  • Air from attic: insulation fibers, dust, critter poop.
  • Air from attached garage: carbon monoxide, gasoline/oil fumes, pesticides, fertilizers.

It starts with the blower door test.

Airtightness must be verified with the building envelope to control IAQ risks. Without stopping unwanted air movement, insulation is ineffective. Compared to measuring a home's thermal envelope (R-value), measuring airtightness is simple.

Blower door thumbnail   The Blower Door Test.  

Posted in Building Envelope

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